Optical imaging systems have traditionally been created using either reflective or refractive optical elements. When refractive optical elements are used within imaging systems, these components often take the form of lenses with one or two curvature radii crafted within a homogeneous material. Use of refractive optical elements introduces various types of geometric and chromatic aberrations onto an optical image plane.
FIG. 1A depicts a lens 110 made of a homogeneous material as well as the spherical aberration 120 produced when imaging an object positioned at infinity. Similarly, FIG. 1B depicts a lens 130 made of a homogeneous material as well as the chromatic aberration 140 produced when imaging an object at positioned at infinity for various wavelengths. Overcoming these aberrations, as well as others such as coma, astigmatism, field curvature and distortion, to produce an image of desired quality often requires the introduction of numerous optical elements cascaded in series, with each element having its own unique geometric and optical properties that contribute to the characteristics of the desired output image. FIG. 2 depicts an example of a lithographic lens imaging system 200 where numerous discrete optical components have been introduced to meet a desired image quality. With each additional optical element added to an imaging system, additional size, weight, cost, and manufacturing complexity are imposed on the final end product.
In some cases, a reduction in the number or size of optical elements within an imaging system can be realized through the introduction of diffractive optical elements as either integrated features of existing refractive or reflective optical elements or completely separate optical elements. Diffractive optical elements can be used to provide functionality that is similar to certain types of refractive or reflective components, but within a smaller material volume.
Typical diffractive optical elements have their own sets of limitations that do not always replace the functionality provided by their refractive counterparts. While typical diffractive elements fashioned from homogeneous materials can be utilized for the correction of certain field aberrations, such as astigmatism, field curvature, and coma, they are less able to correct for chromatic aberrations due to the strong wavelength dispersion properties of most diffractive optical components. However, when combined with the dispersive properties of the substrate materials chromatic aberrations can be corrected.